Contamination indicators of this type are known in the prior art (DE 101 27 021 C1) and are used in hydraulic systems, in which replacement of used filter elements located in the fluid circuit with new filter elements is necessary once a certain degree of contamination has entered to maintain the operational reliability of the system. The known manner in which such contamination indicators function is based on the fact that an increase in pressure in the fluid intake (i.e., the dirty side in the filtering process) is associated with increasing contamination of the filter element. Accordingly, such contamination indicators function based on the fact that these indicators have a movable element such as a plunger or membrane. The plunger or membrane is acted upon by pressure on the dirty side. The deflection of that element provides an indication of the degree to which the filter element is contaminated.
If a hydraulic system is put into operation in a cold state, risk exists that the indicating device of the contamination indicator will signal a degree of contamination that falls below a threshold value that has been predefined as acceptable. This indication is due to the fact that the flow resistance of the filter element is a function of the viscosity of the fluid in the hydraulic oil and the viscosity, in turn, is largely a function of the fluid temperature. In the case of a cold start-up and during the cold phases before reaching the normal operating temperature, which may be in the range of 40° C. for example, the pressure that arises on the dirty side will then signal an insufficient degree of contamination.